This invention relates to opto-electronic scale-reading apparatus for use in measuring relative displacement of two members. In such known apparatus e.g. British Patent No. 1,504,691 the scale comprises a diffraction grating co-operating with at least one other grating on a read head to produce interference fringes which move relative to the read head during a said displacement of the members, and said measurement is a count of said fringes. It is clear that such a scale has to be of diffraction quality, i.e. the accuracy and reliability of the measurement depends on such parameters as the regularity of the spacing of the scale marks, the sharp definition of the edges of the marks, and the freedom of the scale from scratches and like imperfections. Such a scale can be expensive to produce and protect especially when the scale has to be relatively long.
It is among the objects of this invention to overcome or reduce this difficulty.
It is also known to increase the number of signals obtainable from any two adjacent marks of the scale by phase quadrature interpolation. Known scale-reading apparatus can be subject to phase errors and consequent interpolation errors. It is optionally an object of this invention to overcome or reduce this difficulty.